Method and apparatus for initializing filter coefficients in an echo canceller

ABSTRACT

The filter coefficients of an echo cancelling arrangement having a near echo canceller and a distant echo canceller are initialized in accordance with a method and apparatus wherein the following steps are performed: 
     transmission of at least two training sequences each comprising a pair of complementary S and C sequences having autocorrelation functions such that, if added, their sidelobes cancel, each S and C sequence being followed by a variable time interval Δ, determined as a function of the measured distant echo delay τ so that a distant echo produced in response to a training sequence occurs immediately after the near echo produced in response to by a following training sequence; 
     after the first training sequence transmitted, calculation of correlation signals giving the correlation between the received signal and reference signals derived from an S sequence and a C sequence respectively during the first half and the second half of a training sequence of a duration T; 
     application of a delay T/2 to the correlation signal formed on the basis of the S sequence; 
     production of a sum signal of said delayed correlation signal and the correlation signal formed on the basis of the C sequence; and 
     on the basis of said sum signal, extraction of the coefficients of the echo cancellers during determined time windows.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for initializing filter coefficients,used in an echo-cancelling arrangement incorporated in a transceiverequipment to cancel an echo signal occurring in the receive path inresponse to a signal applied to the transmit path and consisting of asubstantially undelayed near echo and a delayed distant echo, saidecho-cancelling arrangement operating at a given sample rate andcomprising a near echo canceller receiving a signal D subjected to thephase variations of the transmit carrier and a distant echo cancellerreceiving the said signal D subjected additionally to a delay which issubstantially equal to the measured distant echo delay τ end echo, thismethod being intended for the initialization of the filter coefficientsof the near and distant echo cancellers.

2. Description of the Prior Art

It is known that echo cancellers are adaptive devices which are formedwith the aid of filters having adjustable coefficients and which areincorporated in data-transmission modems connected to a two-waytransmission circuit in order to cancel automatically undesirable echoesoccurring in the one-way receive path in response to the signal appliedto the one-way transmit path. Conventional echo cancellers are generallydesigned to cancel echo signals which are not delayed or relativelylittle delayed, occurring on national and international terrestrialcircuits.

However, international communications are being increasingly conductedvia communication satellites. In a circuit of this kind, including asatellite link between two radio-relay stations, there may be producedin the receive path of a modem a near echo which is not or littledelayed and is generated in the part of the circuit preceding satellitelink, as well as a distant echo which is generated in the part of thecircuit after the satellite link and which is therefore subject to aconsiderable delay τ, depending particularly on the wave-propagationtime in the satellite link. Since the satellite used may or may not begeostationary and since the terrestrial circuit may differ according tothe connections, it can be estimated that in the international switchednetwork the delay τ of the distant echo may assume values rangingbetween approximately 220 and 630 ms.

To cancel the echo signal consisting of a near echo and a distant echo,which each have a relatively short duration of the order of 10 ms orseveral tens of ms but which are separated by a large time interval ofthe order of the delay τ, it is an advantage to use the echo-cancellingarrangement having the configuration described above and known from thearticle by Stephen B. Weinstein, entitled "A Passband Data Driven EchoCanceller for Full Duplex Transmission on Two-Wire Circuits", andpublished in the journal IEEE Transactions on Communications, Vol.COM-25, No. 7, July 1977, pp. 654-666. This configuration comprises anadaptive transversal filter, which receives a signal from the transmitpath directly and which provides a copy of the near echo when itscoefficients are suitably adjusted, and another adaptive transversalfilter, which receives the signal from the transmit path subject to adelay equal to the measured distant echo delay and which delivers a copyof the distant echo when its coefficients are suitably adjusted. Bysubtracting from the received signal the sum of the signals leaving thetwo filters, the near and distant echoes in the receive path arecancelled. This configuration, which necessitates a, at least rough,previous measurement of the delay τ of the distant echo, has theadvantage of using adaptive filters whose complexity is notunreasonable.

For adjustment of the coefficients of the two adaptive filters aftermeasurement of the delay τ of the distant echo, the article by Weinsteinreferred to above suggests using the gradient algorithm, even during atraining period preceding the full duplex transmission of the usefuldata. With this algorithm the coefficients are adjusted iteratively andtend asymptotically towards their optimum values, leading to a somewhatslow convergence of the two echo cancellers.

SUMMARY OF THE INVENTION

The present invention has for its object to provide a method for therapid aquisition of the filter coefficients of the near echo cancellerand the distant echo canceller, after previous measurement of thedistant echo delay time τ which may be effected by any desired method,e.g. that described in the previously mentioned article by Weinstein orthat described in the applicants' copending U.S. patent application Ser.No. 562,612, filed Dec. 19, 1983 now U.S. Pat. No. 4,577,309.

The invention is based on the idea of generating echoes with the aid ofcomplementary "Golay" sequences and each followed by a time interval Δcontrolled as a function of the measured delay τ. These complementarysequences, described in an article by Golay ("Complementary series"; IRETransactions, Vol. IT-17, April 1961, pp. 82-87), have aperiodicautocorrelation functions such that, if added, the side lobes of thesefunctions cancel. The time interval Δ is controlled with the aid of thedelay τ in such a way that the near and distant echoes are producedwithin predetermined time intervals, always separate and continguous, soas to permit, by calculation of the correlation between the transmittedsequences and the received signal, determination of the filtercoefficients of the two echo cancellers.

The method according to the invention is characterized in that itcomprises at least the following steps:

(a) application to the transmit path of a training signal consisting ofat least two consecutive training sequences each comprising a pair ofcomplementary sequences S and C of a same duration d, having aperiodicautocorrelation functions whose main lobes have the same sign and thesidelobes have substantially the same absolute value and opposite signs,each S and C sequence being followed by a time interval of a variableduration Δ, determined as a function of the measured delay τ so that thedistant echo produced in response to an S or C sequence in each trainingsequence appears within a predetermined time interval of a followingtraining sequence, immediately after the time interval for theappearance of the near echo produced in response to the S or C sequenceof the said following training sequence;

(b) during the duration of each training sequence transmitted after thefirst sequence,

calculation of correlation signals giving the correlation between asignal derived from the received signal and sampled at the same samplerate and reference signals constituted respectively, during the durationof an S sequence and the following duration Δ, by the conjugate value ofthe said signal D applied to the echo cancellers during a sequence Sand, during the duration of a C sequence and the following duration Δ,by the conjugate value of the said signal D applied to the echocancellers during a C sequence,

application of a delay d+Δ to the correlation signal formed during theduration of an S sequence and the following duration Δ,

production of a sum signal of the said delayed correlation signal andthe correlation signal formed during the duration of a C sequence andthe following duration Δ; and

(c) routing of the said sum signal to the near echo canceller and thento the distant echo canceller during two consecutive time intervalsfollowing a C sequence and during which this sum signal constitutes inseries form the coefficients of the near echo canceller and thecoefficients of the distant echo canceller.

In communication systems including satellite links, once a connectionhas been established, it is generally not permissible to have periods ofsilence of a duration Δ which may exceed hundreds of ms. During thesetime intervals Δ, therefore, it is necessary to transmit fill-insignals, while at the same time ensuring that these fill-in signals donot produce parasitic signals during the time intervals in which thecoefficients of the echo cancellers are being formed. To avoid theseparasitic signals, the time intervals Δ in the transmitted trainingsequences comprise, in one variant of the method according to theinvention, fill-in signals such that in two consecutive trainingsequences fill-in signals A and A are used during the time intervals Δfollowing the two S sequences and fill-in signals B and B during thetime intervals Δ following the two C sequences, these fill-in signalsbeing such that A+A=0 and B+B=0. A sufficient number of trainingsequences is transmitted to enable the sum signals of the correlationsignals to be calculated during the duration of an even number oftransmitted training sequences, and such sum signals, derived twice theduration of a training sequence, are accumulated to form a resulting sumsignal which is routed to the near echo canceller and then to thedistant echo canceller and represents the coefficients of these echocancellers.

Features of the invention will be more fully appreciated from thefollowing description of an examplary embodiment when considered inconjunction with the accompanying drawing.

DESCRIPTION OF THE DRAWING

FIG. 1 shows the diagram of a modem incorporating near and distant echocancellers and the coefficient-initializing apparatus for carrying outthe method according to the invention;

FIGS. 2a through 2h show time diagrams illustrating the method accordingto the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The modem provided with an echo-cancelling arrangement and shown in FIG.1 comprises a transmit path 1 and a receive path 2 coupled to a two-waytransmission path via a (hybrid) coupling circuit 4.

The transmit path 1 is connected to a data source 5 which provides abaseband signal B in which the data may change value at a clock rate Hsupplied by a clock generator 7. It will be assumed that phasemodulation, if necesary combined with amplitude modulation, is used inthe modem. In the case of two-state (0°-180°) phase modulation, thebaseband signal B is a real signal. In the case of four- or eight-statemodulation, signal B is a complex signal which undergoes at each period1/H phase jumps corresponding to the data to be transmitted. Signal B isapplied to a phase modulator circuit 6 in which its phase is incrementedat each period 1/H by the phase variation Δφ (during this period 1/H) ofthe carrier used for transmission. In these practical cases ofmodulation, the signal D supplied by circuit 6 is complex, even in theevent that the baseband signal B is real, since the latter has beensubject to the phase variations of the carrier. The complex signal D isapplied to a band-pass filter 8 for complex signals whose passband iscentred on the frequency of the carrier used for transmission. Filter 8thus supplies the analog modulated carrier signal which is applied tothe transmit access of coupling circuit 4. The modulation rate of thecarrier is determined by the clock frequency H. In the case of, forexample, a standardised modem using eight-phase modulation, themodulation rate is 1600 Baud and the frequency of the carrier is 1800Hz; with these values the complex signal D may assume any phase of theeight multiples of π/4 between 0 and 7π/4.

At the receive access of coupling circuit 4 there should appear only thecarrier signal modulated with data in a remote modem, transmitted overtransmission path 3 and intended for processing in a receiver 9, whosefunction is to restore the transmitted data. In fact, when a usefulsignal is transmitted in the direction towards the remote modem viatransmit path 1, unwanted echo signals produced particularly in thetwo-wire/four-wire coupling circuits of the transmission path may appearat the receive access of coupling circuit 4 and seriously interfere withthe restoration of the data by receiver 9. As has been explained, whenthe transmission path 3 includes a satellite link, the unwanted echosignal may simultaneously include a non-delayed near echo generatedbetween the local modem and the satellite link and a distant echogenerated between the satellite link and the remote modem. These twotypes of echo are of substantially the same duration, being at mostseveral tens of ms; but the distant echo has, in relation to the nearecho, a delay which may vary, for example, between 220 and 630 ms.

To achieve economically the cancellation of an echo signal formed by anear echo and a distant echo it is possible to use a cancellingarrangement processing the data signal D and having the configurationshown in FIG. 1. As has been explained, this signal D, modified by thephase variations of the carrier, is complex and the echo-cancellingarrangement is designed to process complex signals. It will be assumed,for the sake of simplicity, that the signal received in the modem issampled at the modulation rate H, which implies that the data signal Dprocessed by the echo-cancelling arrangement is also sampled at rate H.It is, in fact, well known that, if, in order to cancel the echo signalthe received signal has to be sampled with a sampling rate H' which is amultiple of H(H'=qH) so as to satisfy Shannon's theorem, it issufficient to use q identical echo subcancellers, each operatingseparately at the sampling rate H, with a time shift of 1/(qH).

The echo-cancelling arrangement in FIG. 1 includes a near echo canceller10 comprising in particular a memory 10-1 performing the function of adelay line, a memory 11 performing the function of a delay line, and adistant echo canceller 12 comprising in particular a memory 12-1performing the function of a delay line. The three delay lines 10-1, 11and 12-1 are arranged in cascade and receive the signal D.

The near echo canceller 10 comprises a calculation circuit 10-2 whichforms the weighted sum of the samples of signal D, stored in delay line10-1, with complex weighting coefficients stored in a memory 10-3. Delayline 10-1 produces a delay τ₁ which is substantially equal to themaximum duration of the near echo. Delay line 11 produces a delay τ-τ₁such that the samples of signal D arrive at the input of delay line 12-1with a delay which is substantially equal to the delay τ of the distantecho. This delay τ is measured by any known method such as, for example,that described in the above-mentioned article by Weinstein or thatdescribed in the French Patent Application No. 82 22 124, and theinformation relating to the delay τ resulting from the measurement isassumed to be stored in a memory 13. The distant echo canceller 12comprises a calculation circuit 12-2 which forms the weighted sum of thedelayed samples of signal D, stored in the delay line 12-1, with complexweighting coefficients stored in memory 12-3. Delay line 12-1 produces adelay τ₂ which is substantially equal to the maximum duration of thedistant echo and which is of the same order of magnitude as τ₁.

The near echo canceller 10 and the distant echo canceller 12 formtransversal filters with complex coefficients whose output signals ε_(p)and ε_(l), calculated in the calculation circuits 10-2 and 12-2, areapplied to an adder 14. The signal ε_(p) +ε_(l) leaving adder 14 isapplied to the (-) input of a difference circuit 15. This differencecircuit 15 is inserted via its (+) input and its output into the receivepath 2 between coupling circuit 4 and receiver 9. The weightingcoefficients of near echo canceller 10 and distant echo canceller 12,which are stored in memories 10-3 and 12-3, have to be adjusted so thatthe signals ε_(p) and ε_(l) supplied by the transversal filters of theseecho cancellers are practically equal to the near echo signal ε_(p) andthe distant echo signal ε_(l) appearing in the receive path 2. Theeffect of this is that the signal ε_(p) +ε_(l) resulting from the nearand distant echoes is practically cancelled in the output signal fromdifference circuit 15.

The adjustment of the filter coefficients of the near and distant echocancellers is generally effected by successive iterations on a basis ofthe gradient algorithm so as to minimize the mean square value of theoutput signal from difference circuit 15. However, with this methodadvocated in the above-mentioned article by Weinstein, the time neededfor initialization of the coefficients when the arrangement is startedup is necessarily long since the coefficients tend asymptoticallytowards their optimum values. The present invention provides a methodpermitting the rapid calculation of the coefficients of the near anddistant echo cancellers and one which can be used during the starting-upof the echo-cancelling arrangement.

In order, using the method according to the invention, to generate nearand distant echoes to be used to calculate these coefficients, abaseband signal B is transmitted via transmit path 1 with the aid of asuitable data generator 5, which baseband signal B consists of at leasttwo consecutive training sequences having the structure and propertieswhich will be described with the aid of a time diagram 2a in FIG. 2. Inthis diagram, there are shown, for example, three training sequences A₁,A₂ and A₃ with the same duration T, transmitted successively starting atthe originating instant t=0. Each sequence A₁, A₂ or A₃ comprises a pairof complementary sequences S and C, with the same duration d and eachconsisting of a certain number of bits occurring at the modulation rateH. For example, each sequence may consist of 64 bits occurring at a rateof 1600 Hz, corresponding to a duration d of 40 ms. These complementaryS and C sequences have aperiodic autocorrelation functions with mainlobes of the same sign and sidelobes substantially of the same absolutevalue and with opposite signs; the interesting property of this pair ofcomplementary sequences S and C is that, if their aperiodicautocorrelation functions are added, the two main lobes will reinforceeach other while the sidelobes practically eliminate each other.

It was assumed above, in order to concentrate thought, that thecomplementary sequences S and C formed a binary and therefore realsignal. But it is also possible to use complementary S and C sequencesforming a complex signal and possessing the same properties with regardto their aperiodic autocorrelation functions.

Each S or C sequence is followed by a time interval of variable durationΔ which is determined as a function of the measured delay τ of thedistant echo, as will be explained with the aid of diagram 2b associatedwith diagram 2a. In diagram 2b the time intervals p are shown followingthe start of each of the S and C sequences in the training sequences A₁,A₂ and A₃. These time intervals p have a duration D over which, at most,the impulse response of the path of the near echo produced by a Diracimpulse occurring at the start of an S or C sequence extends. The timeintervals Δ are determined so that the impulse response of the path ofthe distant echo produced by such a Dirac impulse at the start of thefirst sequence A₁, for example, occurs during the following sequences A₂or A₃, in the time intervals l of duration D immediately following thetime intervals p. Thus the distant echoes produced by Dirac impulsesoccurring at the start of the S and C sequences of the first sequence A₁may occur in the intervals l of the second sequence A₂, with a delayτ=τ₂ in relation to the instant of occurrence of these Dirac impulses.The distant echoes produced by the Dirac impulses of the first sequenceA₁ might also occur in the course of the time intervals τ in the thirdsequence A₃, with a delay τ=τ₃, or in the course of other followingsequences which are not shown.

The practical determination of the duration Δ in accordance with themeasured delay τ of the distant echo is performed in a device 16, whichreceives the information relating to the delay τ contained in memory 13and which supplies data generator 5 with information characterizing theduration Δ. Generator 5 is arranged to modify, as a function of thatinformation, the durations Δ in the training sequences.

The operation of device 16 accords with the following considerations:the time interval Δ comprises a fixed part with a duration 2D equal tothe sum of the duration of the intervals p and l and a variable partwith a duration ε such that there results:

    Δ=2D+ε

It will be readily deduced from the indications applied to diagrams 2aand 2b that:

    T=2(d+2D+ε)

    τ=kT+D

(k=integer)

Depending on whether the distant echo resulting from a Dirac impulse atthe start of the first sequence A₁ is in the second sequence A₂ or inthe third sequence A₃ or in a fourth sequence A₄, not shown, the valuesk=1 or k=2 or k=3 are found.

On the other hand, the variable part ε of the time interval Δ must bedetermined with a step equal to the modulation interval, or 1/H, where His the modulation rate. It is therefore permissible to write:

    ε=m/H

(m=integer)

Using the above relations giving T, τ and ε, we readily obtain:

    τ=2k(d+2D)+(2km/H)+D

Taking, for example,

for the duration of an S or C sequence: d=40 ms,

for the duration of an interval p or l: D=30 ms,

for the modulation rate: H=1600 Hz,

we obtain for the delay τ expressed in ms:

    τ=200 k+1.25 km+30                                     (1)

For each measured delay τ the parameters k and m have to be chosen suchthat equation (1) is satisfied. Table I below lists the values of k tobe chosen for several ranges of the possible values of the measureddelay τ. The corresponding range of the values of m is also given foreach range of τ.

                  TABLE I                                                         ______________________________________                                        230 ≦ τ < 430                                                                   k = 1         0 ≦ m < 160                                 430 ≦ τ < 630                                                                   k = 2         0 ≦ m < 80                                  630 ≦ τ ≦ 780                                                            k = 3         0 ≦ m ≦ 40                           ______________________________________                                    

For each value of the measured delay τ it is possible to obtain a valuefor m satisfying equation (1), making it possible to form the variablepart ε of the time intervals Δ, where ε=m/H, and finally the timeinterval Δ itself.

It will be a simple matter for a person skilled in the art to conceiveof a device 16 which, via logical methods and known calculating methods,makes it possible to find, corresponding to each measured delay τ, acouple of values for k and m characterizing the duration of the timeintervals Δ. These two values characterizing this duration may betransmitted to data generator 5 for the transmission of suitabletraining sequences.

After the transmission of the first training sequence A₁ possessing theappropriate duration characteristic Δ, processing of the received signalappearing at the receive access of coupling circuit 4 is carried out inaccordance with the method according to the invention. This processingbegins at the start of the training sequence in which the distant echoproduced by the first training sequence A₁ appears. Thisstart-of-processing sequence is A₂ or A₃ depending on whether τ=τ₂ or τ₃(i.e. k=1 or 2). It will be assumed henceforth that processing begins atthe start of the second training sequence A₂.

With the method according to the invention, such processing consistsfirst of all in forming the correlation signal between the complexversion of the received signal and a reference signal which consistseither of the conjugate value S_(p) * of a sequence S_(p) or of theconjugate value C_(p) * of a sequence C_(p), the sequences S_(p) andC_(p) being respectively complementary S and C sequences which have beensubjected to the phase variations Δφ of the carrier, i.e. the sequencessupplied by circuit 6 in response to the S and C sequences. As shown bydiagram 2c associated with diagram 2a, the reference signal is S_(p) *for the duration of transmission of an S sequence and the duration Δimmediately following this transmitted S sequence; the reference signalis C_(p) * for the duration of transmission of a C sequence and theduration Δ immediately following this transmitted C sequence. It may beobserved that the reference signal thus formed is shown in diagram 2conly for the duration of the processing commencing in the example chosenat the start of the second sequence A₂.

The sequences S_(p) and C_(p) delivered by circuit 6 may be written:

    S.sub.p =S exp(j2πf.sub.c t)

    C.sub.p =C exp(j2πf.sub.c t)

where f_(c) is the frequency of the transmit carrier and 2πf_(c) trepresents the phase of this carrier, which is variable with time.

It can be shown that, if the phase of this carrier is the same at thestart of each training sequence, the sequence S_(p) and C_(p), like thereference sequences S_(p) * and C_(p) *, are complementary like theoriginal sequences S and C, i.e., if their aperiodic autocorrelationfunctions are added, the two main lobes reinforce each other while thesidelobes practically eliminate each other. This phase condition of thecarrier is ensured in FIG. 1 by a synchronizing signal S_(y), producedin generator 5, in order to reset to a fixed value φ₀, at the start ofeach training sequence, the phase of the sequences S_(p) and C_(p)supplied by circuit 6.

The above-mentioned processing can be carried out, for example, as shownin FIG. 1. The received signal is taken from receive path 2 and appliedto a circuit 50 which comprises a 90° phase-shifter in order to form theimaginary component of the received signal, circuit 50 delivering acomplex signal consisting of the received signal as real component andthis imaginary component. The received complex signal thus formed isapplied to a sampling circuit 17 to be sampled at the sampling rate H.The received signal thus sampled is applied to a routing circuit 19which is operated by a control signal K₁ so as to occupy its position sduring the time intervals d+Δ in which the reference signal is asequence S_(p) * and to occupy its position c during the time intervalsd+Δ in which the reference signal is a sequence C_(p) *. The controlsignal K₁ is produced by a control circuit 18 on the one hand startingfrom the modulation rate H and on the other starting from theinformation characterizing the variable duration Δ.

Depending on whether routing circuit 19 is in its position s or itsposition c, the sample received complex signal is applied to shiftregister 20 or 21, both of which receive shift pulses of frequency H.These registers have a number n of elements corresponding to theduration d of a transmitted S or C sequence, thus n=64 elements for d=40ms and H=1600 Hz. On the other hand, the n elements of the referencesequences S_(p) * and C_(p) * are stored respectively in memories 22 and23. A control signal K₂ shown in diagram 2d makes it possible to supplyin parallel at the outputs of memories 22 and 23 the n elements of thesequences S_(p) * and C_(p) * during a certain number of trainingsequences following the first sequence A₁. With the signal K₂ in diagram2d, these bits appear during the two training sequences A₂ and A₃. Thecontrol signal K₂ is produced in control circuit 18.

The elements of the sequence S_(p) * appearing at the outputs of memory22 and the complex samples of the received signal appearing in parallelat the outputs of register 20 are applied to a calculation circuit 24which calculates the sum of the products of these elements and thesesamples so as to form the correlation signal E_(s). A correlation signalE_(c) is calculated in the same way by a calculation circuit 25 from theelements of the sequence C_(p) * appearing at the outputs of memory 23and the complex samples of the received signal appearing at the outputsof register 21.

It will be readily appreciated that the correlation signals E_(s) andE_(c) could also be calculated by means of a single calculation devicesuch as 24 jointly with a shift register 20 permanently connected to theoutput of sampling circuit 17 and to a memory 22 alternately supplying asequence S_(p) * and a sequence C_(p) *. The calculation device willthen alternately supply the signal E_(s) and the signal E_(c) whichshould be distributed over two paths, as in FIG. 1.

Assuming a situation in which no signal is transmitted during the timeintervals Δ in the training sequences, diagrams 2e and 2f represent thetime intervals during which the contribution of the near and distantechoes to the correlation signals E_(s) and E_(c) appears. According todiagram 2e, the contribution of the near echo to the signal E_(s)appears during a time interval p' with a duration D following each Ssequence transmitted as from the second training sequence A2; thecontribution of the distant echo to the signal E_(s) appears during atime interval l' with a duration D following each time interval p'.According to diagram 2f, the contribution of the near echo to the signalE_(c) appears during a time interval p" with a duration D following eachC sequence transmitted as from the second training sequence A₂ ; thecontribution of the distant echo to the signal E_(c) appears during atime interval l" with a duration D following each time interval p".

The correlation signal E_(s) is delayed by a duration d+Δ, half of theduration T of a training sequence, with the aid of a delay circuit 26which receives from circuit 16 the information on the variable durationΔ and which produces the delay d+Δ varying as a function of Δ. Thedelayed correlation signal E_(s) and the correlation signal E_(c) areadded with the aid of an adder 27. The result is the sum signal E_(s)+E_(c) shown in the diagram 2g, which signal occurs during the same timeintervals p" and l" as signal E_(c) in diagram 2f. Thanks to thecomplementarity property of reference sequences S_(p) * and C_(p) *whose summed aperiodic autocorrelation functions form a function ofwhich only the main lobe is not zero, the sum signal E_(s) +E_(c)represents, during the time intervals p", the impulse response of thepath of the near echo and, during time intervals l", the impulseresponse of the path of the distant echo excluding the path producingthe delay τ. Since in fact the sum signal E_(s) +E_(c) is sampled at thesampling rate H, the samples of the impulse response of the near echopath, i.e. the coefficients of the near echo canceller in series, areobtained during the time intervals p", and the samples of the impulseresponse of the distant echo path, i.e. the coefficients of the distantecho canceller in series, are obtained during the time intervals l".

In the case considered hitherto, in which no signal is transmittedduring the time intervals Δ, it would be possible to transmit only thetwo training sequences A₁ and A₂ and to extract the coefficients of thetwo echo cancellers from the signal E_(s) +E_(c) formed during thesequence A₂ with the aid of suitable time windows. But it may be useful,in order to improve the signal-to-noise ratio, to accumulate the signalE_(s) +E_(c) during several periods T of the training signal beforeextracting the coefficients of the echo cancellers from it. Diagram 2h,for example, shows the signal 2 (E_(s) +E_(c)) resulting from theaccumulation of the E_(s) +E_(c) signal formed during the two trainingsequences A₂ and A₃. In this case it is possible to obtain thecoefficients of the near echo canceller in series during a time windowcoinciding with the time interval p" in the sequence A₃, and to obtainthe coefficients of the distant echo canceller in series during a timewindow coinciding with the time interval l" in the sequence A₃.

The operation of accumulating the sum signal E_(s) +E_(c) is carried outin FIG. 1 with the aid of an accumulator 28 connected to the output ofadder 27 and achieved with the aid of an adder 29 and a delay circuit 30arranged as shown in the figure. Circuit 30 produces a delay equal tothe period T of the training signal and is controlled by the variableduration Δ since T is a function of Δ. The accumulation is effectedduring a number of periods T defined by the signal K₂, e.g. two periodsin the case illustrated by the diagrams in FIG. 2.

The output of accumulator 28 is connected on the one hand to coefficientmemory 10-3 of the near echo canceller via a gate 31 shown in the formof an interrupter contact and on the other hand to coefficient memory12-3 of the distant echo canceller via a gate 32. Gates 31 and 32 makeit possible to obtain time windows during which the coefficients of thenear and distant echo cancellers are extracted. These gates 31 and 32are controlled respectively by the control signals K₃ and K₄ generatedby control circuit 18 and making these gates conducting respectivelyduring the time intervals p" and l" of the sequence A₃.

The case considered so far is that in which no signal is transmittedduring the time intervals Δ in the training sequences. But satellitecommunications are effected by time division and in many cases it isimpossible to tolerate such periods of silence, whose duration Δ dependson the particular connection. It is therefore necessary to transmitfill-in signals during the time intervals Δ. However, the correlationsignals E_(s) +E_(c) then comprise terms depending on these fill-insignals, resulting in parasitic signals in the time windows during whichthe coefficients of the near and distant echo cancellers are extracted.

These parasitic signals can be avoided by using fill-in signals suchthat, in two successive training sequences, the fill-in signals for thetwo corresponding time intervals Δ add up to zero. For example, asdiagram 2a shows, the first and second time intervals Δ in the firstsequence A₁ can be filled respectively by any signals A and B. But thefirst and second time intervals in the second sequence A₂ have then tobe filled by signals A and B such that A+A=0 and B+B=0. The fill-insignals A and B will be used for the third sequence A₃ and so on.Signals A and B can be identical and formed very simply, for example, byan alternating sequence of +1 and -1.

With these fill-in signals, A, A, B and B, parasitic correlation termsare obtained which are superimposed on the useful correlation terms andwhich are:

during the duration of the second sequence A₂, the result of thecorrelations of A with S_(p) * and of B with C_(p) *;

during the duration of the third sequence A₃, the result of thecorrelations of A with S_(p) * and of B with C_(p) * .

If the result of the correlations performed during the duration of thesetwo sequences A₂ and A₃ is accumulated the parasitic correlation termsare cancelled out and in the time windows defined by the time intervalsp" and l" of sequence A₃, the coefficients of the near and distant echocancellers are obtained free of the parasitic signals brought about bythe fill-in signals.

In the event that fill-in signals as defined above are used, the periodof the training signal is 2T, i.e. twice the duration of a sequencecomprising the fill-in signals A, B or A, B. To improve thesignal-to-noise ratio, therefore, the result of the correlations can beaccumulated during a duration which is a multiple of 2T, i.e. an evenmultiple of the duration T of a sequence.

The method according to the invenion and the corresponding apparatus inFIG. 1 have been described for the case that the echo-cancellingarrangement processes the signal D with a sample rate equal to themodulation rate H. As has been indicated above, it may be decided tohave the echo-cancelling arrangement operate with a signal D sampled ata sample rate H'=qH, i.e. a multiple of H. In this case theecho-cancelling arrangement is made up of q branches each operating atthe sampling rate H on samples of the signal D distributed in time andeach composed like the arrangement in FIG. 1, i.e. of a near echosub-canceller similar to 10, of a delay line similar to 11 and of adistant echo sub-canceller similar to 12. In order to initialize thecoefficients of the q near echo sub-cancellers and the q distant echosub-cancellers, it is necessary, using the method according to theinvention, to sample the received signal with a sampling rate H'=qH,then to distribute as a function of time the samples of the receivedsignal over q initialization arrangements each similar to theinitialization arrangement formed by elements 19 to 32. Each of theseinitialization arrangements operates at the sample rate H to provide thecoefficients of the near and distant echo sub-cancellers of a branch.

The method according to the invention such as it has been describeduntil now makes it possible to obtain in a single step the complexcoefficients of the near and distant echo cancellers, thanks to the useof the complex version of the received signal to form the correlationsignals E_(s) and E_(c). However, with a variant of the method accordingto the invention, it is possible to obtain complex coefficients withoutforming the complex version of the received signal, thus rendering itpossible to avoid using a circuit 50 with a 90° phase-shifter.

With this variant, the initialization of the coefficients is effected intwo steps. In the first step the chosen succession of training sequencesforming a baseband signal B is generated with the aid of generator 5. Incircuit 6, this signal B is subjected to the phase variations Δφ of thein-phase transmit carrier so as to form the signal D. By using thereceived signal directly (i.e. by omitting circuit 50), complexcoefficients K_(1p) are obtained, exactly as has been explained, for thenear echo canceller and complex coefficients K_(1l) are obtained for thedistant echo canceller. In the second step the same baseband signal B isgenerated with the aid of generator 5. However, this signal B issubjected in circuit 6 to the phase variations Δφ of thequadrature-phase transmit carrier so as to form the signal -jD. By usingthe received signal directly and by using the same reference signalsS_(p) * and C_(p) * as in the first stage to form the correlationsignals E_(s) and E_(c), complex coefficients K_(2p) are obtained forthe near echo canceller and complex coefficients K_(2l) for the distantecho canceller. The coefficients K_(p) and K_(l) to be used for the nearand distant echo cancellers are obtained by summing the coefficientsformed at the end of the two steps, namely: ##EQU1##

The advantage of omitting a 90° phase-shifter in forming circuit 50 inthis variant is offset by the fact that the coefficient-initializationtime is doubled.

What is claimed is:
 1. For use in a data transceiver having a transmitpath and a receive path and which includes an echo-canceller arrangementfor cancelling an echo occurring in the receive path in response totransmission by said transceiver of a phase modulated digital signalapplied to the transmit path, said echo comprising a substantiallyundelayed near echo signal and a delayed echo signal; saidecho-cancelling arrangement operating at a given sample rate andcomprising a near-echo canceller to which the transmitted signal issupplied and a distant echo canceller which meausres a delay τ of thedistant echo and to which the transmitted signal is supplied with adelay substantially equal to the measured distant echo delay τ, the nearand distant echo cancellers each comprising a filter having adjustableweighting coefficients; a method for initializing the filtercoefficients of the near and distant echo cancellers, comprising thesteps of:(a) application to and transmission by the transmit path of thetransceiver of a digital training signal D comprising at least twoconsecutive training sequences each of which includes a pair ofcomplementary sequences S and C of the same duration d, said sequences Sand C having aperiodic autocorrelation functions whose main lobes havethe same sign and whose sidelobes have substantially the same absolutevalue and opposite signals, each S and C sequence being followed by atime interval of a variable duration Δ determined from the measureddistant echo delay τ so that the distant echo signal produced inresponse to an S or C sequence in each training sequence occurs during afollowing training sequence immediately after occurrence of a near echosignal in the receive path of the transceiver in response to one of theS and C sequences in the following training sequence; (b) during eachtraining sequence transmitted after the first training sequence.derivingcorrelation signals E_(S) and E_(C) respectively representing thecorrelation, between an echo signal received in response to atransmitted S sequence and an echo signal received in response to atransmitted C sequence, respectively, which echo signals are sampled atsaid sample rate, and respective reference sequences S_(p) * and S_(c) *; the reference sequence S_(p) * representing, during said transmitted Ssequence and the following time interval Δ, the conjugate value of saidtraining signal D during said transmitted S sequence; and the referencesequence S_(c) * representing, during said transmitted C sequence andthe following time interval Δ, the conjugate value of said trainingsignal D during said transmitted C sequence; delaying the correlationsignal E_(S) by a delay d+Δ, deriving the sum of said delayedcorrelation signal E_(S) and said correlation signal E_(C), said sum ofthe correlation signals serially representing during a first intervalthereof an impulse response of the echo path for application to the nearecho canceller and serially representing during a second intervalthereof an impulse response of the echo path for application to thedistant echo canceller; and (c) routing said sum of the correlationsignals to the near echo canceller during said first interval of saidsum of the correlation signals to control formation of the filtercoefficients of the near echo canceller and to the distant echocanceller during said second interval of said sum of the correlationsignals to control formation of the filter coefficients of the distantecho canceller.
 2. A method as claimed in claim 1, in which a sufficientnumber of training sequences are transmitted so that each of thecorrelation signals E_(C) and E_(S) are derived during at least twotransmitted training sequences, the signals E_(C) and E_(S) so derivedduring at least two transmitted sequences being accumulated andthereafter summed to form a sum signal which is switched to the nearecho canceller and to the distant echo canceller to control formation ofthe filter coefficients thereof.
 3. A method as claimed in claim 2, inwhich fill-in signals are transmitted by the transceiver during the timeintervals Δ in the transmitted training sequences, fill-in signals A andA' being transmitted in two consecutive training sequences during thetime intervals Δ following the two S sequences therein, and fill-insignals B and B' being transmitted in said two consecutive trainingsequences during the time intervals Δ following the two C sequencestherein, the fill-in signals being such that A+A'=0 and B+B'=0.
 4. Amethod as claimed in claim 1, wherein the correlation signals E_(S) andE_(C) respectively represent the correlation between the complex form ofthe echo received in response to a transmitted S sequence and the echoreceived in response to a transmitted C sequence, respectively, and saidreference signals S_(p) * and S_(c) *, respectively.
 5. In a datatransceiver having a transmit path and a receive path and which includesan echo-cancelling arrangement for cancelling an echo occurring in thereceive path in response to transmission by said transceiver of a phasemodulated digital signal applied to the transmit path, said echocomprising a substantially undelayed near echo signal and a delayeddistant echo signal; said echo-cancelling arrangement operating at agiven sample rate and comprising a near echo canceller to which thetransmitted signal is supplied and a distant echo canceller whichmeasures a delay τ of the distant echo and to which the transmittedsignal is supplied with a delay substantially equal to the measureddistant echo delay τ, the near and distant echo cancellers eachcomprising a filter having adjustable weighting coefficients; apparatusfor initializing the filter coefficients of the near and distant echocancellers, said apparatus comprising:a data source connected to thetransmit path of the transceiver for transmitting a digital trainingsignal D comprising at least two consecutive training sequences each ofwhich includes a pair of complementary sequences S and C of the sameduration d, the sequences S and C having aperiodic autocorrelationfunctions whose main lobes have the same sign and whose side lobes havesubstantially the same absolute value and opposite signs, each S and Csequence being followed by a time interval of a variable duration Δ;calculating circuit means connected to each of said echo cancellers fordetermining said time interval Δ from the measured distant echo delay τand for applying a signal corresponding to the time interval Δ to saiddata source, the value of the time interval Δ being such that thedistant echo signal produced in response to an S or C sequence in eachtraining sequence occurs during a following training sequenceimmediately after occurrence of a near echo signal in the receive pathof the transceiver in response to one of the S and C sequences in thefollowing training sequence; correlating circuit means connected to thereceive path of the transceiver and to said calculating circuit meansfor deriving, during each transmitted training sequence following thefirst training sequence, correlation signals E_(S) and E_(C)respectively representing the correlation between a signal derived fromthe echo signal received in response to a transmitted S sequence and atransmitted C sequence, respectively, and which is sampled at saidsample rate, and respective reference sequences S_(p) * and S_(c) *; thereference sequence S_(p) * representing, during said transmitted Ssequence and the following time interval Δ, the conjugate value of saidtraining signal D during the transmitted S sequence; the referencesequence S_(c) * representing, during said transmitted C sequence andthe following time interval Δ, the conjugate value of said trainingsignal D during the transmitted C sequence; circuit means connected tosaid correlating circuit means for applying a delay d+Δ to thecorrelation signal E_(S) and deriving the sum of the delayed correlationsignal E_(S) and the correlation signal E_(C), said sum of thecorrelation signals serially representing during a first intervalthereof an impulse response for the echo path for application to thenear echo canceller, and serially representing during a second intervalthereof an impulse response of the echo path for application to thedistant echo canceller; and circuit means connected to said sum derivingcircuit means for routing the sum of the correlation signals producedthereby to the near echo canceller during said first interval of saidsum of the correlation signals and to the distant echo canceller duringsaid second interval of said sum of the correlation signals, each ofsaid first and second intervals of said sum of the correlation signalsfollowing transmission of a C sequence in the transmit path of saidtransceiver; whereby the sums of the correlation signals supplied to thenear and distant echo cancellers control formation of the coefficientsthereof for effecting echo signal cancellation.
 6. Apparatus inaccordance with claim 5, further comprising circuit means connected tosaid sum deriving circuit means for accumulating sum signals producedthereby during a predetermined number of training signal sequences andsupplying accumulated sum signals to said routing circuit means, theaccumulated sum signals constituting the signals for routing to the nearand distant echo cancellers.